What can we learn from the Monte Carlo simulation of the glass transition of polymer melts?

The use of coarse-grained lattice models for the modelling of the glassy freezing in of polymer melts is discussed. The bond-fluctuation model is shown to be a reasonable compromise between simulation efficiency and realistic chemical detail. The potential energy, favoring large values of the length of effective bonds, is at low temperatures in conflict with the tendency for dense packing of the effective monomers and this “geometrical frustration” is responsible for the glass transition of this model. It is shown that many physical properties of the model are in surprisingly close agreement with experiments. Open problems and directions for future research are briefly discussed.     

Motivated attention and prepulse inhibition of startle in rats: Using conditioned reinforcers as prepulses.

In humans, prepulse inhibition (PPI) of startle is greater during attended prestimuli than it is during ignored prestimuli, whereas in rats, most work has focused on passive PPI, which does not require attention. In the work described in this article, researchers developed a paradigm to assess attentional modification of PPI in rats using motivationally salient prepulses. Water-deprived rats were either conditioned to attend to a conditioned stimulus (CS; 1-s, 7-dB increase in white noise) paired with water (CS+ group), or they received uncorrelated presentations of white noise and water (CSo group). After 10 conditioning sessions, startle probes (50 ms, 115 dB) were introduced, with the CS serving as a continuous prepulse. Three experiments examined PPI across a range of prepulse intensities (4-10 dB) and stimulus onset asynchronies (SOAs; 30-960 ms). PPI was consistently reduced in the CS+ group, particularly with a 10-dB prepulse and a 60-ms SOA. Thus, PPI in rats differed between attended and ignored prestimuli, but the effect was reversed in the results of research with humans. A fourth study eliminated the group difference by reversing the CS-water contingency. Methodological and motivational hypotheses regarding the current findings are discussed. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Subjective effects of transdermal nicotine among nonsmokers.

The subjective experience of nicotine, which may be influenced by personality traits as well as environmental factors, may be important for understanding the factors associated with the initiation and maintenance of nicotine dependence. The present study examined the effects of 7 mg transdermal nicotine among a relatively large sample (n = 91; 44 women) of college-aged nonsmokers. Using a placebo controlled, double-blind, within-subjects design, nicotine’s effects were examined at rest and again after participants completed a sustained attention task. Sex and personality factors (Behavioral Inhibition and Behavioral Approach; BIS/BAS Scales; Carver & White, 1994) were examined as potential moderators. Overall, the effects of nicotine were generally modest and unpleasant. In the context of the cognitive task, nicotine increased nausea and negative affect but reduced fatigue, relative to placebo. In contrast, effects of nicotine during the initial 4 hr of patch administration, in which participants were in their natural environments, were moderated by individual differences in behavioral approach. Neither behavioral inhibition nor gender reliably moderated any subjective effects of nicotine. The present work suggests transdermal nicotine exerts only modest, mostly negative effects among nonsmokers. Future work should examine both contextual and personality moderators in large samples of participants who are exposed to nicotine through multiple routes of administration. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Strictly two-dimensional self-avoiding walks: Density crossover scaling

The density crossover scaling of thermodynamic and conformational properties of solutions and melts of self-avoiding and highly flexible polymer chains without chain intersections confined to strictly two dimensions (d = 2) is investigated by means of molecular dynamics and Monte Carlo simulations of a standard coarse grained bead-spring model. We focus on properties related to the contact exponent set by the intrachain subchain size distribution. With R ～ N ^ν being the size of chains of length N and ρ the monomer density, the interaction energy e _int between monomers from different chains and the corresponding number n _int of interchain contacts per monomer are found to scale as with ν = 3/4 and θ₂ = 19/12 for dilute solutions and ν = 1/d and θ₂ = 3/4 for N? g(ρ) ≈ 1/ρ². Irrespective of ρ, long chains thus become compact packings of blobs of contour length with d _p = d ? θ₂ = 5/4 being the fractal line dimension. Due to the generalized Porod scattering of the compact chains, the Kratky representation of the intramolecular form factor F(q) reveals a non-monotonous behavior approaching with increasing chain length and density a power-law slope $F(q)q^d /\rho \approx 1/(qR)^{\theta _2 } $ in the intermediate regime of the wavevector q. The specific intermolecular contact probability is argued to imply an enhanced compatibility for polymer blends confined to ultrathin films. We comment briefly on finite persistence length effects.     

Impulsivity and risk-taking in borderline personality disorder with and without substance use disorders.

Impulsivity and risk taking propensity were assessed in participants with borderline personality disorder (BPD-only; n = 19), BPD and a current or past substance use disorder (BPD-SUD; n = 32), and a matched comparison group (MC; n = 28). Participants were administered behavioral measures of two facets of the multidimensional construct of impulsivity [GoStop, delay discounting task (DDT)], one measure of risk-taking propensity [Balloon Analog Risk Task (BART)], and two self-report measures of impulsivity (i.e., Barrett Impulsiveness Scale, Eysenck Impulsiveness Scale). The BPD-SUD group, but not the BPD-only group, discounted delayed rewards faster than the MC group on the DDT, suggesting that the BPD-SUD/MC group difference may be because of the SUD rather than BPD. In contrast, both the BPD-SUD and BPD-only groups exhibited poorer behavioral response inhibition compared with the MC group, but the two BPD groups did not differ from one another. This finding suggests that the differences in behavioral response inhibition may be because of BPD rather than SUD and that behavioral response disinhibition may be a core feature of BPD. None of the groups differed on the measure of risk-taking propensity (i.e., BART). On self-report questionnaires, the BPD-SUD group reported more impulsivity than the BPD-only group and both BPD groups reported more impulsivity than the MC group. Data from the DDT and self-report measures provide partial support for the hypothesis that BPD individuals with a SUD are more impulsive than BPD individuals without a SUD on some facets of impulsivity (e.g., desire to obtain a smaller immediate reward rather than wait to obtain a larger reward in the future). Results suggest that behavioral response inhibition may be a novel treatment outcome variable for BPD treatment studies. (PsycINFO Database Record (c) 2011 APA, all rights reserved)     

Structure and dynamics of thin polymer films: a case study with the bond-fluctuation model

This paper reports Monte Carlo simulation results of a polymer melt of short, non-entangled chains which are embedded between two impenetrable walls. The melt is simulated by the bond-fluctuation lattice model under athermal conditions, i.e. only excluded volume interactions between the monomers and between the monomers and the walls are taken into account. In the simulations, the wall separation is varied from about one to about 15 times the bulk radius of gyration R_g. The confinement influences both static and dynamic properties of the films: Chains close to the walls preferentially orient parallel to it. This parallel orientation decays with increasing distances from the wall and vanishes for distances larger than about 2R_g. Strong confinement effects are therefore observed for film thicknesses D?4R_g. The preferential alignment of the chains with respect to the walls suppresses reorientations in perpendicular direction, whereas parallel reorientations take place in an environment of high monomer density. Therefore, they have a relaxation time larger than that of the bulk. On the other hand, the influence of confinement on the translation motion of the chains parallel to the walls is very weak. It almost coincides with the bulk behavior even if D≈1.5R_g. Despite these differences between translational and reorientational dynamics, their behavior can be well reproduced by a variant of Rouse theory which only assumes orthogonality of the Rouse modes and determines the necessary input from the simulation.     

高压IGBT应用中用作过电压保护的开关自箝位模式“SSCH”

本文介绍高压SPT、（软穿通）-IGBT器件的开关自箝位模式，并且引入一种现有短路过流保护能力以外的、器件关断过程中的过压保护特性。IGBT结构中的这种新的保护特性将为那些正在寻找可靠、经济的应用IGBT的高压解决方案的系统设计者提供一个全新的前景。